The present invention generally relates to a digital audio signal processing method, and more particularly to a method of processing digital audio signals which provides digital audio processing and mixing capabilities for CD-ROM application systems which deal with input digital audio data of different data recording formats.
There are different data recording standards in which audio data is recorded on a compact disc within a CD-ROM (compact disc-read only memory) application system. For example, a 16-bit linear PCM (pulse code modulation) data recording format used by the CD-DA (compact disc-digital audio) standard, and ADPCM (adaptive differential PCM) data recording formats used by the CD-I (compact disc-interactive) standards and CD-ROM.XA standards.
The sampling rates of the CD-DA, the CD-I ADPCM Level A, the CD-I ADPCM Level B and the CD-I ADPCM Level C are 44.1 kHz, 37.8 kHz, 37.8 kHz and 18.9 kHz, respectively.
In order to convert the digital audio data from the compact disc into analog signals and output the analog signals, conventional D/A (digital-to-analog) converters and low pass filters are used. If the conventional D/A converters and low pass filters are used, it is necessary to provide a plurality of D/A converters with conversion rates that are respectively in conformity with the sampling rates of the above standard formats and a plurality of low pass filters with characteristics that are respectively in conformity with the sampling rates of the above standard formats.
In addition, in the case of the CD-ROM application systems, it may be necessary to perform the mixing of the CD-DA audio data and the CD-I ADPCM audio data and the reproducing of the mixed audio data. In this case, also, the D/A converters with conversion rates in conformity with the sampling rates of the standard formats and the low pass filters with characteristics in conformity with the sampling rates of the standard formats must be provided.
However, if the respective D/A converters and the respective low pass filters for each of the sampling rates of different standard formats are provided in a CD-ROM system, the construction of a complicated circuit is required and the cost of manufacture of that circuit must be high.
Further, if a digital filter circuit for mixing input digital audio signals of different formats mentioned above is provided for each of different channels, a CD-ROM system having such digital filter circuits must have a large size and the cost of manufacture of the CD-ROM system must be high.